4/02/07 FCST: NE/IA/IL/MO/WI

[Jesse Risley]

A developing lower pressure system will move across the eastern central plains today, with a decent southerly flow pushing a frontal boundary north, which will extend from southern Nebraska eastward across northern MO and into southern IA at 00Z. Both the NAM (which appears more aggressive) and GFS show CAPE of at least 1000J/KG. Adequate warm air advection interacting with a favorable LLJ of 45-50 knots should set the stage for some convection late this afternoon as a couple of SFC waves interact with the frontal boundary.

 

[Brandon Sullivan]

IA/IL/MO/NE

Ongoing precip in IA, hopefully will clear out soon.. I really wish DMX would do a sounding and kind of fill in the bubble.. I'm still wondering which event to chase.. Tommorow or today.. Today seems to be somewhat closer to home at least..

Discussion.. Impressive shear values already overspreading the area VIA the 15z MSAS.. Nice H7-H4 vorticity advection taking place.. Two rounds of storms to deal with tonight..

Round One- This afternoon in Northern and Central IA, associated with the warm front, with the nice shear and instabilty in place, all modes of severe appear possible, with a heightened tornado risk immediately near the front....

Round Two- Tonight, associated with warm sector/cold frontal boundary, the threats at this time appears to be linear with large hail/bowing segments. These storms will fire in Nebraska and move Eastward, and maintain strength due to warm sector/favorable dynamics...

Since Today's threat appears to hold more of a tornado potential than tommorow, I problably will end up chasing this one.. I want to see both SWODY 1 and SWODY 2 before I leave around 3 o'clock however..

Jesse, send me a PM. If you want to chase we can head out after 3..

 

[Bill Schintler]

Chase target for Monday, April 2

Chase target:
Shenandoah, IA.

Timing:
Storm initiation: 7 PM CDT.

Storm mode:
Supercell storms early in convective evolution, with impressive storm structure. Hail to golf-ball size will be the primary severe threat.

Synopsis:
UA analysis indicates a number of shortwaves ahead of deepening WRN CONUS trough, the first of which was over SRN WY while a secondary ripple noted on the H7 map and WV imagery was translating through NRN NV. At the SFC, a developing WF was noted from LBF to CDJ with 50F dewpoints along and south of this feature. Meanwhile, dewpoints in the mid-60’s had already worked into NERN OK. An area of steep mid-level lapse rates in excess of -8.5C/km was surging NEWRD out of CO. Ongoing precipitation in IA along and N of I-80 in association with mid-level WAA along with isentropic up-glide along the 295K-300K SFCs, will serve to enhance baroclinicity across the area today. NAM and GFS have initialized well to SFC moisture, WF position, and UA fields; while the 13km RUC was several degrees too low with SFC dewpoints.

Discussion:
Shortwave trough currently over NRN CO/SERN WY will translate ESEWD towards SERN SD while intensifying. Attendant SFC low will develop from WRN/CNTRL WY into NWRN KS by 00Z, and then translate EWD into IA during the overnight hours while slowly weakening. Strengthening WF will extend EWD from the low towards the OMA area, and then ESEWD into SERN IA by 00Z.

Impressive differential advection has commenced. By late afternoon, Td’s near 60F will have surged N into SRN IA/NE while steep mid-level lapse rates will overspread the area from the SW. Most areas will remain capped to SFC-based convection until after 00Z owing to the EML. During the early evening hours, however, a strengthening LLJ with associated isentropic up-glide should assist with the development of storms, which may be SFC-based for an hour or two following initiation. Sufficient instability with MLCAPE’s of 1500J/kg will be juxtaposed with impressive shear parameters with deep-layer (SFC-6km) shear of 45 kts along with SRH (SFC-3km) in excess of 250m2/s2 along the WF. Overnight, a veering LLJ will focus elevated storms which will move across IA and SRN MN.

- bill

 

[Matthew Crowther]

Color me not impressed- I do not understand why SPC has a 5% TOR risk, esp in central IA where it is and will stay quite stable at the surface. The highest Tds I see on the OK mesonet are in the upper 50s to near 60, so mid 50s may be the top we can hope for in the target area. The RUC suggests maybe a small before sunset risk in NW missouri for a few surface based storms, but the bulk of this event will be elevated hailers in the gunge.

 

[Brandon Sullivan]

No offense, but I totally understan why they have the 5 % tor.. Steep lapse rates combined with CAPE of around 1000 J/KG combined with the warm front, and warm sector, and available moisture, would give at least a good supercell risk.. And with the windfields, organized rotating updrafts appear possible.. Again, No offense.. That is just my take on it.. Timing looking less favorable, I may just stay local, and do an overnight thing..

 

[Matthew Crowther]

No offense, but I totally understan why they have the 5 % tor.. Steep lapse rates combined with CAPE of around 1000 J/KG combined with the warm front, and warm sector, and available moisture, would give at least a good supercell risk.. And with the windfields, organized rotating updrafts appear possible.. Again, No offense.. That is just my take on it.. Timing looking less favorable, I may just stay local, and do an overnight thing..

Organized rotating updrafts, quite possible. However, tornadoes seem less likely as most of the storms will be elevated above the stable surface layer. If there is a tornado, it is more likely to be farther south in the warm sector right near the warm front in northern MO or along the IA border, not in central IA where the SPC risk is centered. Another problem for chasers is that storms may very well not form until right at or after dark.

 

[Mike Hollingshead]

Evidently they think it is going to lift further north, where previous runs had suggested. Looks like the 2% goes well into MO and the 5% touches the border, so the space to the north must be for the guess on where it *could* wind up. Warm fronts seem to be one of the more tricky fronts to place as they love to "jump". My pick was Onawa IA but it has some "jumping" to do for that to happen at this point. As for TDs, I'd bet mid-50s would do with it being -17c at 500mb above this front.

 

[Jayson Prentice]

Warm front will be the key factor in where storms are surface based compared to the elevated hailers. Currently the warm front sits along the KS/NE border and into northern MO, with some slight overrunning on it. Dewpoints are surging northwards and should begin to pool along the warm front, with a last surge of moisture with the aid of a strong shortwave coming from the west.

Warm front should continue to move northward through the afternoon hours, thus putting the best risk along it this evening. Expected position of the warm front across Iowa seems best along Tekamah to DMX to DVN line, with best risk likely along the western portion of this. Dewpoints in the upper 50s to a few 60 readings seem plausible. Lift will be quite sufficient with the aid of the shortwave, instability also sufficient with SBCAPE values >1000 j/kg. Wind shear is forecasted to increase and provide ample turning throughout the layer for orgnanized rotation updrafts and thus the large hail threat is especially likely. Tornado threat will be there for a short time period, especially right along the warm front where Low level shear is enhanced.

Target: Guthrie Center, IA -- Initalization ~ 6 PM

 

[Ryan Pfannkuch]

good elevated supercell setup for IA

4-county target area for supercells capable of large hail and MAYBE an isolated TOR in IA AFTER 0z this evening: Cass, Adair, Montgomery,Adams
Including the cities of: Atlantic, Greenfield, Red Oak, Corning

after glancing at the 6z NAM and some of the 6z experimental ESRL RUC data very early this morning...I was not as impressed with today's IA potential than I was from the 0z NAM last night...which had looked much more favorable. So...after going back to bed for several hours and waking up to find the upgrade to 5% TOR and 30% hail probs on the 13z/1630z SWODY1...it's safe to say I needed to re-evaluate matters.

Now that I've had a chance to peruse the 12z NAM/ 15z RUC suite...the kinematic setup still looks quite favorable for supercell structure with 0-6km shear of 50-60kt and very nice veering profiles with loopy hodographs at 0z along the warm front near the IA-MO border. That leaves moisture/instability as the usual suspect...and although the NAM seems to overdue things in this dept. at times...even the 15z RUC brings SBCAPE of 1000-1500 j/kg into the warm front by 0-3z. Just between 12-17z todaythe surface Td at KMCI has jumped from 38 to 54...so I'd have to say there is a legitimate low level moisture surge well underway. And the 40kt SSW 850mb feed into the boundary by 0z is always nice to see as well.

All this being said...I am still not convinced that this looks like a decent tornado setup...with CINH being an issue on the sfc boundary through at least 0z..and the most likely scenario being slightly to considerably elevated hailers based around 800-850mb initiating across southern and/or central IA in the 0-3z timeframe...potentially well north of the surface boundary in an environment characterized by dewpoints in the upper 40s/low 50s at best. At any rate, I am still hanging out in my native IA from the Sat. event...so I will most likely be out and about later this evening hoping for at least a good lightning/ice chunk show...as this looks like the last gasp for decent convection for awhile in these parts.